1. Field of the Invention
The present invention relates to an optical wavelength division multiplex communication network in the technical field of optical communications and an optical wavelength conversion apparatus and a method usable in optical wavelength division multiplex communications and so forth, and particularly, to an optical wavelength conversion apparatus and a method for directly converting a wavelength of an optical signal into another as the optical signal is maintained.
2. Related Background Art
Conventionally, in a distributed Bragg reflector (DBR) type semiconductor with separate multi-electrodes, there has been proposed an optical wavelength conversion apparatus in which part of its active layer is used as a saturable absorber region (see Kondo et al., "Giga-bit Operation of Wavelength Conversion Laser", 1990 International Topical Meeting on Photonic Switching, 13D-9, 1990). FIG. 1A shows the structure of that optical wavelength conversion apparatus. In FIG. 1A, reference numeral 521 designates a first active region, reference numeral 522 designates a saturable absorber region, reference numeral 523 designates a second active region, reference numeral 524 designates a phase adjusting region and reference numeral 525 designates a DBR region.
FIG. 1B shows the operation of the optical wavelength conversion apparatus in FIG. 1A. Its horizontal and vertical axes respectively indicate an injection current into the second active region 523 and the intensity of output light. It is seen therefrom that the current in its oscillation state obtained by increasing the injection current is larger than the current in its non-oscillation state obtained by decreasing the injection current from the oscillation state. As illustrated in FIG. 1B, when the injection current is set at a bias point A, no output light is emitted when no input light is input into the apparatus. When the input light is input into the apparatus, the light absorption coefficient of the saturable absorber region 522 is reduced and the laser hence reaches the oscillation state, emitting the output light. A wavelength of the output light can be varied by controlling currents injected into the phase adjusting region 524 and the DBR region 525, so that the wavelength of the input light can be converted into a desired wavelength of the output.
In that optical wavelength conversion apparatus, since the required time within which the saturable absorber region 522 returns to its initial state after the wavelength conversion operation is dominated by the carrier injection time, the modulation speed of an optical signal for the wavelength conversion operation is normally limited to the order of nanoseconds and high-speed operation is thus impossible.
In order to solve the above problem, there has also been proposed another optical wavelength conversion apparatus which uses the oscillation of light in two mutually-perpendicular polarization modes in a semiconductor laser to convert the wavelength (see Japanese Patent Application Laid-open No. 6(1994)-120595). FIG. 2 illustrates the structure of this optical wavelength conversion apparatus. In FIG. 2, a predetermined polarization mode (which corresponds to one polarization mode of light emitted from a semiconductor laser 601) of input light 612 is selected by a polarization beam splitter device 608, and the selected one is input into the semiconductor laser 601 through a lens 609. The semiconductor laser 601 forms an external resonator cavity between its input facet and a mirror 652 and attains the laser oscillation at a desired wavelength with the selected polarization mode (this desired wavelength is picked up by a wavelength filter 662 with a voltage terminal 672 for controlling the wavelength). In contrast thereto, where the input light 612 does not contain the desired wavelength, the external cavity is formed between the input facet of the semiconductor laser 601 and another mirror 651, and the laser oscillation occurs in the other polarization mode and at a given wavelength selected by a wavelength tunable filter 661.
As discussed above, output light 613 can be obtained when the input light 612 does not contain the desired wavelength. Here, since the oscillation wavelength can be controlled by the wavelength tunable filter 661, as a result, the wavelength of the input light 612 can be converted into the desired wavelength of the output light 613. Though the signals of the input light 612 and the output light 613 are in an inverted relationship with each other, the output light 613 can be readily returned to an original signal of the input light 612. Further, in FIG. 2, reference numeral 602 designates an anti-reflection coating provided on one output facet of the semiconductor laser 601, reference numeral 603 designates an electrode for a current supply, reference numeral 604 designates a polarization beam splitter device for separating two mutually-perpendicular polarization modes emitted from the semiconductor laser 601, reference numeral 610 designates a lens for guiding the light emitted from the semiconductor laser 601 to the polarization beam splitter device 604, and reference numeral 611 designates an output lens for receiving output light 613 from mirror 651.
In that optical wavelength conversion apparatus, however, wavelength tunable filters for each polarization mode are needed, as well as mirrors for constructing the respective external cavities. The large number of optical elements makes the structure complicated.